Embodiments of the present invention relate to a method of transmitting data by charge modulation, and a device for transmitting data implementing this method.
Embodiments of the present invention particularly relate to devices for the contactless transmission of data by inductive coupling, or Radio Frequency Identification devices (RFID), such as those described by the standards ISO/IEC 14443 and ISO/IEC 15693. Embodiments of the present invention particularly relate to contactless data transmission devices of passive type, which are able to extract a supply voltage from an alternating signal supplied by the antenna circuit thereof.
FIG. 1 is a conventional base diagram of a contactless data transmission device DV0 of passive type, provided for emitting data by inductive coupling and charge modulation. For simplicity, such a device will be hereinafter referred to as a “contactless device”. The device DV0 includes a tuned antenna circuit ACT, a charge modulation transistor SW, and a data transmission circuit DSCT. The antenna circuit ACT includes an antenna coil La and a capacitor Ca in parallel with the antenna coil.
The charge modulation transistor SW, here a MOS transistor, has a first conduction terminal (source) connected to a terminal T1 of the antenna circuit ACT and a second conduction terminal (drain) connected to a terminal T2 of the antenna circuit ACT. A rectifying diode Dr is connected between the terminal T1 and the ground GND of the device. A smoothing capacitor Cs and a Zener diode DZ in parallel are connected between the terminal T2 of the antenna circuit ACT and the ground GND.
The contactless device DV0 is activated by a magnetic field FLD oscillating at a working frequency equal to or near a tuning frequency of the antenna circuit ACT. The magnetic field FLD is, for example, emitted by a reader RD1, which is equipped to that end with an antenna coil Lr and applies thereto an excitation signal oscillating at the working frequency. This working frequency is, for example, 13.56 MegaHertz (MHz) in a device complying with one of the aforementioned ISO standards.
By inductive coupling, an alternating antenna signal appears in the antenna coil La of the device DV0. This antenna signal makes a signal VA1 appear on the terminal T1 of the antenna circuit ACT and a signal VA2 appear on the terminal T2 of the antenna circuit ACT.
With reference to the ground GND of the device DV0, the signal VA1 is a half-wave rectified alternating signal and the signal VA2 is a nearly direct signal smoothed by the capacitor Cs and clipped by the diode DZ, for example near 5 Volts (V). The contactless device DV0 is here assumed to be purely passive, and the rectified signal VA1 is used as direct supply voltage Vcc of the data transmission circuit DSCT.
Such a base architecture of a contactless device allows various contactless electronic portable objects to be made, such as contactless chip cards, contactless identification badges, electronic tags, or the like.
To send data DTx to the reader RD1, the data transmission circuit DSCT applies to the transistor SW a binary charge modulation signal Slm1, which is generated according to the data DTx, which is generally coded according to a predetermined coding protocol (NRZ, Manchester, BPSK, or the like). When the signal Slm1 is at “1”, it is equal or substantially equal to the voltage Vcc (ignoring the loss in commutation elements internal to the data transmission circuit DSCT). The transistor SW is then conductive and has a determined series resistor (drain-source resistor RDson). The antenna circuit ACT is therefore short-circuited by the resistor RDson, which is typically set at about several hundred Ohms. The effect of the short-circuit (in actuality a partial short-circuit since the series resistor is not equal to zero) is to modulate the impedance of the antenna circuit ACT, and the modulation of impedance passes on in the antenna signal of the reader RD1 by inductive coupling. The reader may thus, thanks to an adapted filtering circuit, extract the modulation signal from its own antenna signal and deduce therefrom, after demodulation and decoding, the data DTx sent by the device DV0.
FIG. 2A shows the shape of the signal VA1 when the transistor SW is blocked, and the shape of the signal VA1 when the transistor SW is conductive, during a charge modulation period spreading between two instants t1 and t2. The supply voltage Vcc signal is also shown. FIG. 2B shows the shape of the charge modulation signal Slm1, which is at 1 (Vcc, high state) between the instants t1 and t2 and at 0 (ground, low state) outside of the charge modulation periods. Outside of the charge modulation periods, the amplitude of the signal VA1 is at a maximum and the signal VA1 oscillates between a peak value VA1 max and a negative minimal value −Vd, which is equal in absolute value to the threshold voltage Vd of the rectifying diode Dr. The supply voltage Vcc is also at a maximum. During the charge modulation period spreading between the instants t1 and t2, the peak value of the signal VA1 decreases substantially and does not exceed a value VA1min, while the supply voltage Vcc also decreases. Both phenomena are caused by the modification of the amplitude of the magnetic field FLD, caused by the modification of the impedance of the antenna circuit of the reader RD1 in response to the short-circuit applied to the antenna circuit ACT of the device DV0.
The advantage of this structure of contactless device is that the charge modulation depth (i.e., amplitude), that is, the difference between the amplitudes VA1max and VA1min of the alternating signal VA1, is relatively low and preserves the reception of electrical energy during the modulation periods.
This advantage has however proven to become a drawback in applications where a greater charge modulation depth is desired in order to increase the communication distance between the reader RD1 and the contactless device DV0. Indeed, the amplitude of the charge modulation signal detected by the reader RD1 is also a function of the distance between the reader RD1 and the contactless device DV0: the longer the communication distance, the poorer the image of the charge modulation signal mixed to the antenna signal of the reader, because the inductive coupling is lower. In these conditions, the communication distance may only be increased by increasing the depth of charge modulation. To measure the impact of charge modulation in the reader, the variation of the antenna current i in the reader is measured, choosing a determined distance between the reader and the device. Measuring current variation is performed using a series resistor giving a voltage V(i). If a variation of minimal voltage V(i)min is not reached, the modulation depth is considered to be insufficient.
It is therefore desirable to allow the charge modulation depth to be increased without penalizing in an unacceptable way the reception of energy, which is necessary to supply the supply voltage Vcc to a purely passive contactless device or to supply an auxiliary supply voltage to a semi-passive contactless device.